1. Field of the Invention
The invention relates to a device using a chiral smectic liquid crystal which exhibits ferroelectricity and also relates to a display apparatus.
2. Related Background Art
With respect to a display apparatus using a ferroelectric chiral smectic liquid crystal, there have been known display apparatuses in each of which a ferroelectric chiral smectic liquid crystal (hereinafter, referred to as an FLC) is injected in a liquid crystal cell which is constructed by arranging two glass substrates in which transparent electrodes are formed on inner surfaces and an orienting process has been performed so as to face each other while keeping a cell gap of about 1 to 3 .mu.m as disclosed in, for instance, U.S. Pat. Nos. 4639089, 4681404, 4682858, 4712873, 4712874, 4712875, 4712877, 4714323, 4728176, 4738515, 4740060, 4765720, 4778259, 4796979, 4796980, 4859036, 4932757, 4932758, 5000545, and 5007716, and the like.
Among the above background arts, in particular, a device in which the FLC is oriented by forming a chevron structure shown in FIG. 1 has excellent light state under a cross nicols, so that a large enough contrast is obtained. FIG. 1 shows a cross sectional view of an orienting state of the FLC arranged between substrates 11 and 12. An FLC 13 forms a layer 15 comprising a plurality of liquid crystal molecules 14. A plurality of layers 15 are aligned in the same direction and a structure in which the layers 15 are bent occurs. At this time, it is preferable that the major axis of the liquid crystal molecule 14 is inclined for the substrates 11 and 12 at a pretilt angle of, preferably, 5.degree. or more. As for the above aligning state, it is desirable that the orienting process is performed to the substrates 11 and 12 by rubbings 16 and 17 in the same direction.
FIG. 2 is a plan view of the device in which the FLC 13 of the above chevron structure is formed. Reference numeral 21 in FIG. 2 denotes a sealing material to seal the substrates 11 and 12. Although not shown, in the above device, a plurality of a first group of belt-shaped electrodes to apply a voltage are arranged on the substrate 11, and a plurality of second group of belt-shaped electrodes are arranged on the substrate 12 so as to cross the first group of belt-shaped electrodes, thereby forming matrix electrodes. A normal line 22 (on a plane surface) of the layer 15 of the FLC 13 is substantially parallel with the rubbing directions 16 and 17. In the device shown in FIG. 2, the liquid crystal molecules 14 are uniformly tilted to the left (on the plane surface) at an angle of +.theta. (spontaneous polarization is set to a polarity directing from the upper portion of the paper surface to the lower portion).
According to the experiments by the inventors et al., by applying a voltage (for instance, AC voltage of .+-.8 volts at 10 Hz) across the upper and lower electrodes of the above matrix electrodes under such a state, the liquid crystal molecules 14 start flowing toward the right side in the layer 15. When such a voltage is continuously applied for a long time (for instance, 20 to 50 hours), as shown in FIG. 3, a region 31 in which the number of liquid crystal molecules 14 has decreased or which has been depleted occurs in the left side portion. On the other hand, a region 32 in which the number of liquid crystal molecules 14 has increased is formed in the right side portion. The inventors consequently have discovered a problem: that an interference color appears over the whole plane surface of the device and the display quality is lost.
In the case where the liquid crystal molecules 14 in FIG. 2(c) are tilted to the right (on the plane surface) at an angle of -.theta. (spontaneous polarization is set to a polarity directed from the lower portion on the paper surface to the upper portion), it has been also found out that the liquid crystal molecules start flowing toward the left side on the contrary to the above case.